https://leetcode.cn/problems/make-the-string-great/description/
class Solution {
public:
    string makeGood(string s) {
        string ret;
        for(auto ch:s)
        {
            if(!ret.empty()&&tolower(ret.back())==tolower(ch)&&ret.back()!=ch)
            {
                ret.pop_back();
            }

            else
            {
                ret+=ch;
            }
        }
        return ret;
    }
};